Within the freeze-fractured membranes of marginal astrocytes, the orthogonal arrays or "assemblies" of small particles are counted and the area they occupy measured in fetal, newborn, young, and mature rats. Regional differences, i.e. those between dorsal and lateral cerebral cortex and medulla in adult rats are being assessed to provide a base line for any changes brought about by different treatments. The greatest number of assemblies are in the adult dorsal cortex. The least number of assemblies by far are in the 20 day old fetus in all regions of the brain. The number rises appreciably during the first 72 hours. The assemblies within glial scars are also being examined. In order to differentiate glia from neurons in situ and in vitro an immunocytochemical localization of nerve specific enolase (NSE) and non-neuronal enolase (NNE) is proving highly useful. Cells derived from neural crest, such as islet cells of pancreas, adrenal medulla, and parafollicular (calcitonin) cells of the thyroid are also selectively stained by NSE antibody as might be such cells after neoplastic transformation.